Sub-Query in MetaMatrix with NOT IN returns zero results but there should be

Issue

• Running below query should return some rows since there are lot of records from results of main query doesn't exist in the results of sub-query.  But running this query in MetaMatrix is returning 0 rows.

  SELECT a.* 
  FROM ERStudio_Repository_View.DMREPO4.REPO_USER AS a
  WHERE a.unix_nm not in (select b.user_id
                                   from Public_Views.PEOPLE AS b)